A kinetic investigation of mononuclear trans-platinum(II) complexes with mixed amine ligands

Abstract

This study was aimed at investigating the substitution behaviour of mononuclear trans-platinum(II) complexes with mixed amine ligands. The rate of substitution of the chloride moieties from the complexes trans-Pt(NH3)(NH2C2H5)Cl2 (tPt2), trans-Pt(NH3)(NH2C3H7)Cl2 (tPt3), trans-Pt(NH3)(NH2C4H9)Cl2 (tPt4) and trans-Pt(NH3)(NH2C5H11)Cl2 (tPt5), by three nucleophiles, viz. thiourea (TU), 1-methyl-2-thiourea (MTU) and 1,3-dimethyl-2-thiourea (DMTU), was studied by stopped-flow spectrophotometry using a large excess of nucleophile. Pseudo-first-order rate constants (kobs) were measured as a function of nucleophile concentration and temperature. Reactions were first order in both [complex] and [nucleophile] and therefore second-order overall (rate = kobs[complex] where kobs = k2[nucleophile]). The kinetics are consistent with a stepwise mechanism involving rate determining substitution of the first chloride followed by a fast second substitution step, with no intermediates being detected. The reactivity of the complexes was largely dependent on the length of the alkyl chain of the alkylamine moiety of the complexes. Computational modelling using density functional theory calculations showed that an increase in chain length by a methylene unit has no direct electronic consequence on the metal centre but did, however, pose significant steric hindrance on the substitution sites due to the flexibility of the alkyl chains and thus governed the overall reaction pattern. 195Pt NMR kinetic studies established that the mixed amine ligands remain coordinated to the metal centre in the final kinetic product. This implies that mononuclear trans-platinum(II) complexes are resistant to complete substitution of ligands by the incoming thiourea nucleophiles at the reaction sites. The reactions follow an associative mechanism of substitution.